Safety and immunogenicity of ERA strain of rabies virus propagated in a BHK-21 cell line.

The ERA strain of rabies virus was propagated in a baby hamster kidney cell line (BHK-21/C13). The viral titer was 10(1.8) tissue culture infective doses (TCID) higher than that of commercial ERA vaccine. The ERA/BHK-21 vaccine in baits retained titers of 10(6.3) to 10(6.4), TCID when subjected to daily temperature fluctuations from 9 degrees C to 24 degrees C for 21 days. This titer, according to a dose response in laboratory foxes, was still capable of immunizing up to 100% of foxes consuming a bait. The ERA/BHK-21 vaccine, when presented in baits, produced antibodies in 80 to 100% of dogs consuming more than one bait. Duration of immunity in foxes, from feeding the ERA strain rabies virus in baits, as determined by resistance to challenge with virulent virus, was at least 48 months. The vaccine strain retained some pathogenicity for nontarget species. In tests carried out on foxes, raccoons, dogs, cats and cattle, the vaccine did not cause vaccine-induced rabies. One of 14 skunks which consumed four baits developed vaccine-induced rabies, but virus could not be isolated from the salivary glands of this animal. The vaccine, when presented in baits, caused vaccine-induced rabies in 37% of laboratory mice, 3.4% of Microtus and 2.6% of Peromyscus species. Rabies virus could not be isolated from the salivary glands of rodents with vaccine-induced rabies. It was concluded that ERA virus propagated in BHK-21/C13 cells and incorporated in an acceptable bait produced a high titer, stable, immunogenic and safe vaccine for foxes.

Immunization of foxes by the intestinal route using an inactivated rabies vaccine.

Approximately 30% of foxes given two doses of an inactivated rabies antigen delivered directly into the intestinal tract developed an immune response as measured by rabies serum neutralizing antibodies. Seven of ten previously immunized foxes showed an anamnestic response following a booster dose of inactivated rabies antigen delivered to the intestinal lumen. Stomach and particularly intestinal contents were destructive to rabies antigen and virus. This effect could be partially neutralized in vitro by the addition of Questran and soybean trypsin inhibitor. Small enteric coated tablets fed to foxes in a hamburger bolus remained in the stomach for up to 13 hours and therefore would provide a poor vehicle for the delivery of antigen to the intestinal tract.